Electrical circuits requiring high power handling capability while operating at high frequencies, such as radio frequencies (500 MHz), S-band (3 GHz) and X-band (10 GHz), have in recent years become more prevalent. Because of the increase in high power, high frequency circuits, there has been a corresponding increase in demand for transistors which are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads.
To provide increased output power, transistors with a larger gate peripheries have been developed. One technique for increasing the effective gate periphery of a transistor is to provide a plurality of transistor cells that are connected in parallel. For example, a high power transistor may include a plurality of gate fingers that extend in parallel between respective elongated source and drain contacts, as illustrated in FIG. 1.
In particular, FIG. 1 illustrates a metal layout of a conventional transistor structure 10 that includes a gate pad 12, a source pad 22 and a drain pad 32 on a semiconductor structure 20. The gate pad 12 is connected by a gate bus 14 to a plurality of gate fingers 16 that extend in parallel in a first direction (e.g., the y-direction indicated in FIG. 1). The source pad 22 is connected to a plurality of parallel source contacts 26 via a source bus 24, and the drain pad 32 is connected to a plurality of drain contacts 36 via a drain bus 34. Each gate finger 16 runs along the y-direction between a pair of adjacent source and drain contacts 26, 36. A unit cell of the transistor 10 is illustrated at box 40, and includes a gate finger 16 that extends between adjacent source and drain contacts 26, 36. The gate length is dimension of the gate metallization in the x-direction, while the gate width is the distance by which the source and drain contacts 26, 36 overlap in the y-direction. That is, “width” of a gate finger 16 refers to the dimension of the gate finger 16 that extends in parallel to the adjacent source/drain contacts 26, 36. The gate periphery of the device refers to the sum of the gate widths for each gate finger 16 of the device 10.
In addition to adding unit cells, the gate periphery of a multi-cell transistor device may be increased by making the gate fingers wider (i.e., longer in the y-direction). As the gate fingers of a device become wider, however, the high frequency performance of the device may be adversely impacted. In addition, making the gate fingers wider typically means that the gate fingers must handle increased current densities, which can cause electromigration of the gate finger metallization.